それでは

The phrases それでは and では (the abbreviated form) can be used at the beginning of a sentence to express something like "if that's the case", with "that" referring to a previous statement. They're typically used in formal or written language, while それじゃ and じゃ are used in spoken language. Other definitions include "if so" or simply "well then."

日本のオタク文化は世界中に広がり、日本を代表する文化の一つになったという人もいる。では、オタク文化がこのように世界に広がった理由は何だろうか？
There are people who say that Japanese otaku culture has spread throughout the world and become one of Japan's representative cultures. So, what do you think the reason is that otaku culture has spread like this throughout the world?

A: 先生、すみません。実は、金曜日に就職のための面接があって、クラスを休まなくてはいけないので、試験が受けられないんですが。
B: そうですか。じゃ、前の日の木曜日に、研究室に受けに来て下さい。
A: はい。それでは、木曜日の先生のオフィスアワーに研究室にうかがいます。
A: Excuse me, sensei. I actually have a job interview on Friday, so since I have to miss class that day, I won't be able to take the test...
B: Is that so? In that case, on Thursday, the day before, please come to my office to take it.
A: Sure. Then, I'll come to your office during office hours on Thursday.